How old is Gramps? Gramps claims to have worked with Mark Twain, which may sound off, but Samuel Clemens died in 1910. Someone who was 18 in 1910 would be only 48 now in 1940. This bears in keeping in mind, when considering who your Heroes might have known in their backstories...
This page raises other questions. Can a pea shooter/slingshot launch a dart? How great an effect would a small dose of carbolic acid have on an elephant? If a person drank carbolic acid, it would cause acute gastroenteritis (inflammation of the stomach and intestines), leading to vomiting, collapse and death -- but only on a failed saving throw.
This is where game mechanics surrounding poison kind of break down in the game. The effects described always assume human mass; not only should increased Hit Dice allow mobsters to shake off the effects of poison, but should lessen the effect of poison when it does take effect. But how to adjudicate that?
A horse can run up to 30 MPH; if we assume that the wagon halves their movement, that brings us down to 15. So, yes, even if the horses make their skill checks and the elephant fails its check, the elephant would still win. Of course, if the wagon was very light, this might not be the case.
(Scans courtesy of Digital Comic Museum.)